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Abstract:

A test cell and method for stress testing a test specimen including a
first platen and a second platen. Each platen having a loading surface,
an inclined surface, and a longitudinal axis. The inclined surface being
inclined relative to the longitudinal axis at an angle and the inclined
surface having a specimen recess formed therein for receiving a portion
of the test specimen such that when the inclined surface of the second
platen is positioned in a face-to-face relationship with the inclined
surface of the first platen, a shear stress is applied to the test
specimen when an axial load is applied to the first and second platens.
The platens further including fluid ports to subject the test specimen to
fluid flow at various pressures and fluid chemistries and ultrasonic
transducers to determine acoustic, compressional, and shear wave
velocities and in multiple orientations.

Claims:

1. A test cell for stress testing a test specimen, comprising:a first
platen having a loading surface, an inclined surface, and a longitudinal
axis extending from the loading surface to the inclined surface, the
inclined surface of the first platen being inclined relative to the
longitudinal axis of the first platen at an angle, the inclined surface
having a specimen recess formed therein for receiving a portion of the
test specimen, the specimen recess having a base formed parallel to the
inclined surface; anda second platen having a loading surface, an
inclined surface, and a longitudinal axis extending from the loading
surface to the inclined surface, the inclined surface of the second
platen being inclined relative to the longitudinal axis of the second
platen at an angle, the inclined surface having a specimen recess formed
therein having a base formed parallel to the inclined surface of the
second platen and for receiving another portion of the test specimen when
the inclined surface of the second platen is positioned in a face-to-face
relationship with the inclined surface of the first platen such that a
shear stress is applied to the test specimen when an axial load is
applied to the first and second platens.

2. The test cell of claim 1 wherein the first platen has at least two
fluid ports intersecting the base of the specimen recess of the first
platen for selectively injecting a fluid into the specimen recess of the
first platen.

3. The test cell of claim 2 wherein the second platen has at least two
fluid ports intersecting the base of the specimen recess of the second
platen for selectively injecting a fluid into the specimen recess of the
second platen.

4. The test cell of claim 1 wherein the inclined surface of the first
platen and the inclined surface of the second platen are supported in a
spaced apart relationship by the test specimen when the test specimen is
seated in the specimen recesses of the first and second platens.

5. The test cell of claim 4 further comprising:a pliable material disposed
between the inclined surface of the first platen and the inclined surface
of the second platen so as to permit transmission of a confining pressure
to a perimeter of the test specimen while allowing movement of the first
and second platens relative to one another.

6. The test cell of claim 1 wherein the specimen recesses of the first
platen and the second platen are cylindrical in shape.

7. The test cell of claim 1 wherein the first platen has a pair of
acoustic recesses formed in an outer surface of the first platen, the
acoustic recesses having a base formed parallel to the base of the
specimen recess, and wherein the second platen has a pair of acoustic
recesses formed in an outer surface of the second platen, the acoustic
recesses of the second platen having a base formed parallel to the base
of the specimen recess of the second platen and axially aligned with the
acoustic recesses of the first platen when the test specimen is seated in
the specimen recesses of the first and second platens, and wherein the
test cell further comprises:a first pair of ultrasonic transducers
positioned in axially aligned acoustic recesses of the first and second
platens; anda second pair of ultrasonic transducers positioned in the
other axially aligned acoustic recesses of the first and second platens.

8. The test cell of claim 7 wherein the first pair of ultrasonic
transducers is capable of generating compression waves.

9. The test cell of claim 9 wherein the second pair of ultrasonic
transducers is capable of generating shear waves.

10. The test cell of claim 7 wherein the first platen further comprises a
third acoustic recess formed in the outer surface thereof, the third
acoustic recess having a base formed parallel to the longitudinal axis of
the first platen, wherein the second platen further comprises a third
acoustic recess formed in the outer surface thereof, the third acoustic
recess of the second platen having a base formed parallel to the
longitudinal axis of the second platen, the third acoustic recesses of
the first and second platens formed in an opposing relationship to each
other and aligned with a center of the test specimen when the test
specimen is seated in the specimen recesses of the first and second
platens, and wherein the test cell further comprises:a third pair of
ultrasonic transducers positioned in the third acoustic recesses of the
first and second platens.

11. The test cell of claim 10 wherein the third pair of ultrasonic
transducers are capable of generating compression waves.

12. The test cell of claim 1 wherein the angle of the inclined surface of
the first and second platens is in a range from greater than 0 degrees to
less than 90 degrees relative to planes extending perpendicular to the
longitudinal axis of the first platen and the second platen.

13. The test cell of claim 1 further comprising a flexible, fluid
impervious sleeve disposed about the first and second platens when the
test specimen is seated in the specimen recesses of the first and second
platens.

14. A method of stress testing a test specimen, comprising:positioning the
test specimen in a test cell comprising:a first platen having a loading
surface, an inclined surface, and a longitudinal axis extending from the
loading surface to the inclined surface, the inclined surface of the
first platen being inclined relative to the longitudinal axis of the
first platen at an angle, the inclined surface having a specimen recess
formed therein for receiving a portion of the test specimen, the specimen
recess having a base formed parallel to the inclined surface; anda second
platen having a loading surface, a inclined surface, and a longitudinal
axis extending from the loading surface to the inclined surface, the
inclined surface of the second platen being inclined relative to the
longitudinal axis of the second platen at an angle, the inclined surface
having a specimen recess formed therein for receiving another portion of
the test specimen when the inclined surface of the second platen is
positioned in a face-to-face relationship with the inclined surface of
the first platen, the specimen recess of the second platen having a base
formed parallel to the inclined surface thereof; andapplying an axial
load to the first and second platens such that a shear stress is applied
to the test specimen.

15. The method of claim 14 wherein the test specimen is cylindrical in
shape and has a first planar end and a second planar end, and wherein the
method further comprises the step of injecting a fluid into the specimen
recess of the first platen so as to cause the fluid to flow over the
first planar end of the test specimen and be discharged from the specimen
recess.

16. The method of claim 15 further comprising the step of injecting a
fluid into the specimen recess of the second platen so as to cause the
fluid to flow over the second planar end of the test specimen and be
discharged from the specimen recess.

17. The method of claim 14 further comprising the step of injecting a
fluid into the specimen recess of one of the first and second platens so
as to cause the fluid to flow through the test specimen.

18. The method of claim 14 further comprising the step of injecting a
fluid into the specimen recess of the first and second platens so as to
cause the test specimen to be immersed in the fluid.

19. The method of claim 14 wherein the inclined surface of the first
platen and the inclined surface of the second platen are spaced apart,
and wherein the method further comprises the step of applying a confining
pressure to the test specimen.

20. The method of claim 14 further comprising the step of:transmitting
ultrasonic compression waves through the test specimen in a direction
perpendicular to the bases of the specimen recesses of the first and
second platens.

21. The method of claim 20 further comprising the step of:transmitting
ultrasonic shear waves through the test specimen in a direction
perpendicular to the bases of the specimen recesses of the first and
second platens.

22. The method of claim 21 further comprising the step of:transmitting
ultrasonic compression waves through the test specimen in a direction
perpendicular to the longitudinal axis of the first and second platens.

23. The method of claim 14 further comprising the step of:transmitting
ultrasonic compression waves through the test specimen in a direction
perpendicular to the longitudinal axis of the first and second platens.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims benefit of U.S. Provisional Application No.
60/906,054, filed Mar. 9, 2007, which is incorporated herein by reference
in its entirety.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates generally to an apparatus that allows
characterization of material mechanical and physical properties, and more
particularly, but not by way of limitation, to a test cell that is
capable of applying a shear stress to a small test sample of material,
such as rock, bone, and cartilage, and which incorporates the ability to
determine acoustic compressional and shear wave velocities in multiple
orientations while the sample is subjected to an axial load and to fluid
flow at various pressures and various fluid chemistries.

[0004]2. Brief Description of Related Art

[0005]Knowing the effects of fluid exposure and the directional dependency
of mechanical properties such as strength and elastic/poroelastic
coefficients is crucial for well-bore stability analysis, hydraulic
fracturing design, and many other field applications in the oil and gas
industry. Conventional test cells for loading test specimens apply a
generally uniform radial pressure or confining stress and an axial
stress. A Hoek cell, for instance, applies axial stress on the two ends
of a cylindrical specimen, while the radial stress is developed by
pressurizing a hydraulic fluid such as oil, around the cylindrical
surface of the specimen, in a test chamber in which the specimen is held.
The radial stress is angularly uniform in that it is the same in all
radial directions and the only variations possible in relation to
differential stress loading are axial and radial (angularly uniform)
relative to each other.

[0006]Test data from the field has shown that the radial stress, more
usually referred to as the horizontal stress, is defined by two principal
stresses and is asymmetrical. The horizontal stress applied by a Hoek
cell is symmetrical and, as such, not suitable for certain testing, such
as well break-out, shear wave splitting, and fracture propagation
testing. In order that specimens may be tested with asymmetrical,
horizontal stresses, it has been necessary to prepare cubes of test
material which much more accurately reflect the principal stresses
encountered in an actual three dimensional situation. Such cuboid samples
are, however, more difficult and expensive to prepare and test.
Furthermore, cuboid samples generally cannot be prepared from the
cylindrical test specimens normally obtained by conventional coring
techniques used for sample recovery, e.g., in the petroleum industry.

[0007]To this end, a need exists for a test cell and method capable of
subjecting a test sample to different applied stress states, and fluid
circulation for any desired time of exposure, while the dynamic elastic
moduli can be simultaneously acquired and monitored applying a shear
stress to a test sample while also applying an axial load as well. It is
to such a test cell and method that the present invention is directed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0008]FIG. 1 is a sectional view of a test cell constructed in accordance
with the present invention.

[0009]FIG. 2 is a perspective view of a cylindrical test specimen of rock.

[0010]FIG. 3 is a cross-sectional view of a first platen and a second
platen.

[0014]FIG. 7 is a cross-sectional view of the test cell shown disposed in
a test chamber and connected to a load plunger.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

[0015]Referring now to the drawings, and more particularly to FIG. 1, a
test cell 10 constructed in accordance with the present invention is
shown with a cylindrical test specimen 12 (FIG. 2) positioned therein.
While the test cell 10 is shown in FIG. 1 containing a rock sample, it
should be understood that the test cell 10 also has application on any
porous or non-porous materials, in particular for bioengineering and
biomechanics applications, such as bone characterization and bone
weakening, cartilage elastic and plastic yields, while using the same set
ups (in metal or in transparent composite material). One of the
advantages of the test cell 10 is that it permits the testing of material
samples that are significantly smaller in size than conventional rock and
bone core samples.

[0016]Referring now to FIGS. 1 and 3, the test cell 10 includes a first
platen 14 and a second platen 16. The first platen 14 is shown to be
positioned above the second platen 16 when the first and second platens
14 and 16 are arranged in a vertical orientation, as is common in
conducting stress tests. However, it will be appreciated that the present
invention is not limited to the orientation of the first and second
platens 14 and 16. Each of the first and second platens 14 and 16, by way
of example, may be a one inch diameter steel rod approximately two inches
in overall length. However, it will be appreciated that the first and
second platens 14 and 16 may be constructed in a variety of shapes and
sizes and fabricated of a variety of materials. As best shown in FIG. 3,
the first platen 14 has a loading surface 18 and an inclined surface 20.
The loading surface 18 is preferably perpendicular to a longitudinal axis
22 of the first platen 14. The inclined surface 20 is inclined relative
to the longitudinal axis 22 of the first platen 14 at an angle 24. The
inclined surface 20 is provided with a specimen recess 26 for receiving a
portion of the test specimen 12. The specimen recess 26 is preferably
cylindrical in shape with a base 28 and oriented in a direction
perpendicular to the inclined surface 20 with the base 28 parallel to the
inclined surface 20.

[0017]Similarly, the second platen 16 has a loading surface 30 and an
inclined surface 32. The loading surface 30 is preferably perpendicular
to a longitudinal axis 34 of the second platen 16. The inclined surface
32 is inclined relative to the longitudinal axis 34 of the second platen
16 at an angle 36. The inclined surface 32 is provided with a specimen
recess 38 for receiving another portion of the test specimen 12. The
specimen recess 38 is preferably cylindrical in shape with a base 40 and
oriented in a direction perpendicular to the inclined surface 32 and the
base 40 is parallel to the inclined surface 32.

[0018]Referring again to FIG. 1, with the test specimen 12 positioned in
the specimen recesses 28 and 40 of the first and second platens 14 and
16, respectively, such that the inclined surfaces 20 and 32 of the first
and second platens 14 and 16 are in a face-to-face relationship, the
inclined orientation of the specimen recesses 26 and 38 permit a shear
stress to be applied to the test specimen 12 when an axial load is
applied to the loading surfaces 18 and 30 of the first and second platens
14 and 16 in a manner to be described below. To this end, the specimen
recesses 26 and 38 are each machined to a specified diameter and a
specified depth for use with a test specimen of a specified size to
prevent rotation of the test specimen 12 within the specimen recesses 26
and 38. Preferably, the test specimen 12 and the specimen recesses 26 and
38 are sized to cause the first platen 14 and the second platen 16 to be
supported in a spaced apart relationship by the test specimen 12 when the
test specimen 12 is seated in the specimen recesses 26 and 38 to permit
the first and second platens 14 and 16 to move relative to one another
when an axial load is applied. By way of example, with a test specimen
having a diameter of 0.793 inches and a depth of 0.282 inches, the
specimen recesses 26 and 38 are preferably machined to have a diameter of
about 0.80 inches and a depth of about 0.90 inches. Also, it will be
appreciated that while the angles 24 and 36 of the inclined surfaces 20
and 32 of the first and second platens 14 and 16 have been illustrated as
being approximately 45 degrees relative to a plane 42 and a plane 44,
respectively, extending perpendicular to the longitudinal axis 22 of the
first platen 14 and the longitudinal axis 34 of the second platen 16,
other inclination angles ranging from greater than 0 degrees to less than
90 degrees may be used provided stress and strain analysis is modified
accordingly.

[0019]To enable the test specimen 12 to be subjected to fluid flow at
various pressures and various fluid chemistries, each of the first platen
14 and the second platen 16 is provided with a pair of fluid ports. More
specifically, the first platen 14 is provided with two fluid ports 46 and
48 which preferably intersect the base 28 of the specimen recess 26 at or
near the perimeter of the base 28 at the highest and lowest points,
respectively. Likewise, the second platen 16 is provided with two fluid
ports 50 and 52 which preferably intersect the base 40 of the specimen
recess 38 at the highest and lowest points, respectively. The fluid ports
46 and 50 may serve as injection ports, while the fluid ports 48 and 52
may serve as exit ports. The fluid ports 46-52 allow the circulation of a
test fluid or series of test fluids across each face of the test specimen
12. This configuration allows three different modes of circulating fluid:
(1) same upstream and downstream pressures can be used on both end of the
specimen so that circulation fluid is flowed only on the two end
surfaces; (2) different but uniform pressures can be used on the top and
bottom surfaces, inducing fluid flow through the specimen; or (3) a
combination of both flow types.

[0020]Fluid pore pressure and fluid flow can simultaneously be applied to
the porous media, thus measuring strength and material parameters and
variations of these when exposed to fluids with different chemistries,
and while in contact with the fluids. A commercially available precision
flow rate syringe pump with high pressure (10,000 psi) capability
circulates the test fluid across the face of the test specimen 12. The
test fluid circulated across the faces of the test specimen 12 is
collected from the fluid discharge port(s) and may be analyzed using
standard laboratory equipment to detect alterations in the chemical
composition of the test fluid resulting from chemical reaction of the
fluid with the test specimen 12. Determination of the mechanical
properties of the test specimen 12 following contact with the test fluid
allows an evaluation of the sensitivity of the sample material to various
fluids or an evaluation of the effect of the duration of fluid exposure
time if tests are performed with different fluid circulation times (for
example, one hour, three hours, twenty-four hours).

[0021]Referring now to FIG. 6, shown is a schematic representation of the
test cell 10 illustrating the first and second platens 14 and 16
functioning to house three pairs of ultrasonic transducers 54a and 54b,
56a and 56b, and 58a and 58b, to allow for measurements of P-wave
velocity propagates in a direction perpendicular to the top and bottom of
the test specimen 12, Vp90; S-wave velocity propagate in a direction
perpendicular to the top and bottom of the test specimen 12, Vs90; and
P-wave velocity propagate at angles 24 and 36, Vp45. These measured
velocities can be used to obtain the full set of anisotropy stiffness
coefficients and thus allow for monitoring of the changes of the
elastic/poroelastic properties of the test specimen 12 when subjected to
different applied stress states. The actual position of the ultrasonic
transducers 54a and 54b, 56a and 56b, and 58a and 58b, will be described
in detail below.

[0022]As shown in FIG. 4, the first platen 14 has a pair of acoustic
recesses 60a and 60b formed in an outer surface of the first platen 14.
The acoustic recesses 60a and 60b each have a base 62 formed parallel to
the base 28 of the specimen recess 26. As shown in FIG. 5, the second
platen 16 has a pair of acoustic recesses 64a and 64b (only the recess
64a in view in FIG. 5) formed in an outer surface of the second platen
16. The acoustic recesses 64a and 64b of the second platen 16 also have a
base 66 formed parallel to the base 40 of the specimen recess 38 of the
second platen 16 and axially aligned with the acoustic recesses 60a and
60b of the first platen 14 when the test specimen 12 is seated in the
specimen recesses 28 and 40 of the first and second platens 14 and 16.
The acoustic recesses 60a, 60b and 64a, 64b are positioned such that a
portion of the acoustic recesses 60a, 60b, and 64a, 64b is immediately
behind the specimen recesses 28 and 40 and aligned along a diameter of
the specimen recesses 26 and 38 perpendicular to a line through the fluid
ports 46, 48 and 50, 52. In one embodiment, the acoustic recesses 60a,
60b and 64a, 64b are drilled to a depth to allow 0.2 inches of steel
between the base of the specimen recess and the base of the acoustic
recess.

[0023]Referring to FIGS. 3 and 4, the first platen 14 is further provided
with a third acoustic recess 68, and the second platen 14 is provided
with a third acoustic recess 70. The third acoustic recess 68 of the
first platen has a base 72 formed parallel to the longitudinal axis 22 of
the first platen 14, and the third acoustic recess 70 of the second
platen 16 has a base 74 formed parallel to the longitudinal axis 34 of
the second platen 16. In one embodiment, the third acoustic recesses 68
and 70 are machined to a maximum depth of about 0.050 inches on the outer
surface of the first and second platens 14 and 16 aligned with the long
point of the inclined surface 20 and 32, respectively, and positioned a
distance back from the long point such that the third acoustic recesses
68 and 70 are positioned in an opposing relationship to each other and
aligned with a center of the test specimen 12 when the test specimen 12
is seated in the specimen recesses 26 and 38 of the first and second
platens 14 and 16.

[0024]The ultrasonic transducers 54a and 54b are mounted in the acoustic
recesses 60a and 64a, the ultrasonic transducers 56a and 56b are mounted
in the acoustic recesses 60b and 64b, and the ultrasonic transducers 58a
and 58b are mounted in the acoustic recesses 68 and 70. In one
embodiment, the ultrasonic transducers are preferably specific frequency
piezo-electric crystals (600 kilohertz, 1 megahertz, or 2 megahertz) with
a diameter of 0.25 inches. The ultrasonic transducers 54a and 54b
preferably generate compression waves, the ultrasonic transducers 56a and
56b shear waves, and the ultrasonic transducers 58a and 58b compression
waves.

[0025]The formation of a test cell stack 80 will now be described with
reference to FIG. 7. Loading spacers 82 and 84 are positioned on the
loading surfaces 18 and 30 of the first and second platens 14 and 16. The
loading spacers 82 and 84 are preferably steel rods one inch in length
and one inch in diameter which have two recesses (not shown) machined
partially through the length thereof. One recess is positioned near the
outer surface of the loading spacer. The second recess is positioned near
the center of the loading spacer with a slot extending to the outer edge
of the loading spacer. The midline of the recess matches the position of
the fluid ports 46-52 of the first and second platens 14 and 16 to permit
tubing 85 to be connected to the fluid ports 46-52.

[0026]A soft metallic acoustic coupling disc (not shown) with etched flow
channels and fluid passage ports is positioned in the base of the
specimen recesses 26 and 38 of both the first and second platens 14 and
16 to provide acoustic coupling between the platens 14 and 16, and the
test specimen 12. The circumferential outer surface of the test specimen
12 preferably is lightly coated with lubricant, such as a petroleum
jelly, to minimize friction during specimen installation. The test
specimen 12 is positioned in the specimen recess 38 of the second platen
16 which, along with its corresponding loading spacer 84, is positioned
on a V-block (not shown) and clamped in position using a clamping device
typically provided with the V-block. The first platen 14 and its
corresponding loading spacer 82 are positioned on an identical sized
V-block and positioned such that the test specimen 12 will enter the
specimen recess 26 of the first platen 14.

[0027]The first and second platens 14 and 16 are moved axially toward each
other until the test specimen 12 is seated in the specimen recesses 26
and 38. A bar clamp (not shown) with one fixed clamp end and one
adjustable clamp end is brought in contact with a loading surface of the
loading spacers 82 and 84 and tightened to hold the composite sample
stack in position and seat the test specimen 12 in the specimen recesses
26 and 38. Excess lubricant is removed from the edge of test specimen 12
exposed in the gap between the first and second platens 14 and 16. A thin
strip of flexible rubber self-vulcanizing tape (not shown) is applied to
the exposed surface of the test specimen 12 in the gap between the first
and second platens 12 and 14. The gap between the first and second
platens 14 and 16 is then filled with a pliable material 86, such as a
commercially available hot melt glue or other suitable material, to
permit the transmission of a confining pressure to the test specimen 12
while allowing relative movement between the first platen 14 and the
second platen 16. After the pliable material 86 has solidified, the bar
clamp is removed. A strip of flexible rubber self-vulcanizing tape (not
shown) is wound around the outer surface of each of the first and second
platens 14 and 16 near the inclined surfaces 20 and 32 and the loading
surfaces 18 and 30 to create pressure sealing areas on the outer surface
of the first and second platens 14 and 16.

[0028]A fluid impervious heat shrink tube 88 is positioned about the
platens 14 and 16 and a soft temper lock wire (not shown) is wrapped
around the heat shrink tube 88 at the positions of the self-vulcanizing
tape and tightened to seal the heat shrink tube 88 against the tape.
Another section of heat shrink tubing 88 is positioned around the
assembled stack 80 including the loading spacers 82 and 84 and shrunk
using a commercially available heat gun.

[0029]Electrical lead wires (not shown) with solid body end pins are
pushed through the heat shrink tubing 88 to connect to the wiring
connections on the ultrasonic transducers 54a, 54b, 56a, 56b, and 58a,
58b. The perforations in the heat shrink tube 88 are sealed with sealing
material, such as hot melt glue and a small second layer of heat shrink
tubing over the area of the perforations.

[0030]The assembled stack 80 is positioned in a test chamber 90 and held
in position by plastic strips around tie-down loops on the base of the
test chamber 90 and a fluid supply tubing is attached to the second
platen 16. All tubing lines are connected inside the test chamber 90. All
electrical leads are connected to appropriate electrical connections
inside the test chamber 90 and tested for electrical short circuits. The
test cell stack 80 is assembled.

[0031]As shown in FIG. 7, the test cell 10 is positioned on the base of
the test chamber 90. A loading disc (not shown), may be positioned on the
top of the loading spacer 82 to increase the contact area between the
test cell 10 and a loading plunger 92 which passes through the top of the
test chamber 90. The plunger 92 is positioned with a flat surface in
contact with the test cell 10.

[0032]The entire test chamber 90 is installed in a loading frame (not
shown) to apply force to the test cell 10. The test chamber 90 has the
capability of being pressurized with fluid, such as water or oil, to
provide a confining pressure to the test specimen 12 and heated to a
specified temperature while in the loading frame to apply a hydrostatic
force on the test cell 10 and thereby simulate in situ conditions. The
loading force is increased by advancing the plunger 92 into the test
chamber 90 at a constant rate of movement until the test specimen 12
experiences mechanical failure. Tests can be performed at various
confining pressures within the operating limits of the test chamber 90 to
characterize the mechanical properties of the test specimen 12.

[0033]From the above description it is clear that the present invention is
well adapted to carry out the objects and to attain the advantages
mentioned herein as well as those inherent in the invention. While
presently preferred embodiments of the invention have been described for
purposes of this disclosure, it will be understood that numerous changes
may be made which will readily suggest themselves to those skilled in the
art and which are accomplished within the spirit of the invention
disclosed and as defined in the appended claims.